Pressure-modulated energy level for pacing pulses

ABSTRACT

Techniques for pacing the heart of a patient as a function of a pressure value make use of a pressure monitor that receives a signal from a pressure sensor in the heart. The pressure monitor measures a pressure value. The pressure monitor may, for example, estimate the pulmonary artery diastolic pressure if the pressure sensor is located in the right ventricle, or calculate the mean central venous pressure if pressure sensor is located in the right atrium. The energy level of the pacing pulses delivered to the patient&#39;s heart by a pacemaker is modulated as a function of the pressure value. Modulating the energy level of the pacing pulses modulates the cardiac output of the patient&#39;s heart.

FIELD OF THE INVENTION

[0001] The present invention relates generally to cardiac pacemakers andcardiac monitoring, and more particularly to cardiac pacemakers thatwork in cooperation with pressure monitors.

BACKGROUND

[0002] Heart failure refers to the heart's inability to keep up with thedemands made upon it. Congestive heart failure refers to an inability ofthe heart to pump an adequate amount of blood to the body tissues.Because the heart is unable to pump an adequate amount of blood, bloodreturning to the heart becomes congested in the venous system.

[0003] In a healthy heart, the heart pumps all of the blood that returnsto it, according to the Frank-Starling law. Increased venous returnleads to increased end diastolic volume, which causes increased strengthof contraction and increased stroke volume. In addition to intrinsiccontrol according to the Frank-Starling law, a healthy heart is subjectto extrinsic control, such as stimulation by the sympathetic nervoussystem to enhance contractility.

[0004] In a patient experiencing congestive heart failure, intrinsic andextrinsic control mechanisms may not function properly, and consequentlythe heart may fail to pump an adequate amount of blood. A conditionknown as cardiac decompensation is used to describe heart failure thatresults in a failure of adequate circulation.

[0005] Failure of the left side of the heart is generally more seriousthan the failure of the right side. On the left side of the heart, bloodreturns from the pulmonary system and is pumped to the rest of the body.When the left side of the heart fails, there are consequences to boththe pulmonary system and to the rest of the body. A patient withcongestive heart failure may be unable to pump enough blood forward toprovide an adequate flow of blood to his kidneys, for example, causinghim to retain excess water and salt. His heart may also be unable tohandle the blood returning from his pulmonary system, resulting in adamming of the blood in the lungs and increasing his risk of developingpulmonary edema.

[0006] Causing more blood to be expelled from the heart, i.e.,increasing cardiac output would reduce the damming of blood in the lungsand the congestion of blood in the venous system caused by heartfailure. In addition to pharmacological therapies to increase cardiacoutput, some patients with congestive heart failure benefit from animplanted pacemaker. A pacemaker rhythmically generates pacing pulsesthat spread throughout the heart to drive the atria and ventricles. Atypical pacemaker monitors the electrical activity of the patient'sheart and provides pacing pulses to cause the heart to beat at a desiredrate, such as sixty beats per minute. Because cardiac output depends inpart on heart rate, increasing the pacing rate of a pacemaker has beenused as a method of increasing cardiac output.

[0007] Existing methods for increasing or optimizing cardiac output mayinvolve modulation of a variety of parameters associated with a pacingprogram other than the pacing rate. For example, some existing methodsmodulate atrial escape interval, atrioventricular (A-V) delay,sequential mode of operation, refractory period, pacing pulse energy,pulse amplitude, or pulse width, which is sometime referred to as pulseduration. The energy level of a pacing pulse is a function of severalparameters, including pulse amplitude and pulse width.

[0008] For example, Nakayama, et. al., “High Output Ventricular PacingIncreased the Cardiac Output,” EUR.J.C.P.E., Vol. 6, No. 1, June 1996,reported that high voltage amplitude ventricular pacing increasedcardiac output as measured by Doppler echocardiogram and cardio-thoracicratio. Cardiac output was higher when paced at high voltage amplitude,4.2 or 5.0 volts, than at low voltage amplitude, 2.5 volts. Nakayama,et. al., concluded that the increase in cardiac output was due to thesynchronous contraction of the ventricle caused by a larger fieldstimulation area due to high voltage pacing.

[0009] Because the condition of a patient may change between visits to aphysician, and because the patients need for increased cardiac outputmay also vary as a result of the demand caused by the patient'sactivity, it is desirable to monitor the patient's need for increasedcardiac output continuously. Some existing methods use implanted devicesthat can estimate cardiac output and control a pacemaker to modulatepacing parameters to maximize cardiac output in a feedback mechanism.For example, U.S. Pat. No. 5,891,176, issued to Bornzin, disclosesmeasuring mixed venous oxygen saturation, blood flow, or ventricularpressure as an estimate of cardiac output, and modulating pacingparameters such as atrial escape interval, A-V delay, and sequentialmode of operation to maximize mixed venous oxygen saturation, bloodflow, or ventricular pressure. U.S. Pat. No. 6,314,323, issued toElkwall, discloses integrating a measured ventricular pressure curveduring systole, using the integration result as an estimate of cardiacoutput, and modulating pacing parameters such as A-V delay, stimulationrate, refractory period, stimulation pulse energy, duration andamplitude to maximize the integrated value. These existing methods,however, may not accurately estimate the need for increased cardiacoutput, or may require complicated devices and methods to estimate theneed for increased cardiac output. These problems may cause lesseffective treatment of the symptoms of cardiac decompensation, or mayincrease complexity, expense, and power consumption of an implantabledevice.

[0010] Examples of the above referenced existing techniques and/ordevices may be found in the issued U.S. Patents listed in Table 1 below.TABLE 1 U.S. Pat. No. Inventor Issue Date 6,314,323 Elkwall Nov. 6, 20015,891,176 Bornzin Apr. 6, 1999 5,626,623 Kieval et al. May 6, 19975,368,040 Carney Nov. 29, 1994

[0011] All patents listed in Table 1 above are hereby incorporated byreference herein in their respective entireties. As those of ordinaryskill in the art will appreciate readily upon reading the Summary of theInvention, Detailed Description of the Preferred Embodiments and claimsset forth below, many of the devices and methods disclosed in thepatents of Table 1 may be modified advantageously by using thetechniques of the present invention.

SUMMARY OF THE INVENTION

[0012] The present invention has certain objects. That is, variousembodiments of the present invention provide solutions to one or moreproblems existing in the prior art with respect to treatment of cardiacdecompensation utilizing a pacemaker. Such problems may include, forexample, the inaccuracy or complexity of existing systems and methodsfor identifying a need for increased cardiac output and modulatingpacing parameters to increase cardiac output to meet this need. It is anobject of the present invention to provide a more accurate and lesscomplicated system and method for identifying a need for increasedcardiac output and modulating pacing parameters to increase cardiacoutput to meet this need. In particular, it is an object of the presentinvention to treat cardiac decompensation by modulating the energy levelof pacing pulses delivered by a pacemaker with a signal based uponintra-cardiac pressures.

[0013] The present invention has certain features. In particular,various embodiments of the present invention may include a pacemakerthat can deliver pacing pulses to the heart at a variety of differentenergy levels, and that is responsive to a control signal to modulatethe energy level of pacing pulses. The energy level of a pacing pulse isa function of several parameters, including pulse amplitude and pulsewidth. Therefore, in some embodiments of the present invention, theenergy level of pacing pulses delivered by the pacemaker may bemodulated by varying, among other things, the amplitude or width of thepacing pulses.

[0014] Various embodiments of the present invention may also include apressure sensor that detects pressure within the heart, and a pressuremonitor that receives a pressure signal from the pressure sensor. Insome embodiments of the present invention, the pressure monitorprocesses the pressure signal, and measures a pressure value that isindicative of a need for increased cardiac output. In particular, insome embodiments, the pressure monitor may differentiate a pressuresignal to, for example, estimate the pressure in the right ventriclethat causes the pulmonary valve to open. In some embodiments, thepressure monitor may receive pressure signals from the atria to, forexample, calculate the beat-to-beat mean central venous pressure (CVP).In some embodiments, the measured pressure value is then used to causethe pacemaker to adjust the energy level parameters of pacing pulsesdelivered to the patient's heart.

[0015] Various embodiments of the invention, therefore, may include aprocessor that receives a signal from the pressure monitor thatindicates the measured pressure value, and generates a control signal tocontrol the pacemaker to adjust the energy level parameters of pacingpulses delivered to the patient's heart as a function of the measuredpressure value. In some embodiments, the processor may select one ormore energy level parameter values by comparing the measured pressurevalue to a look-up table of pressure values and associated parametervalues. In other embodiments, the processor might select the energylevel parameter values by applying one or more equations that relatepressure values to parameters. The look-up table or equations may bestored in memory. The look-up table or equations may, for example, bereceived via remote distribution link or RF telemetry.

[0016] In some embodiments, the processor may receive programming from aphysician via remote distribution link, RF telemetry, or otherwise viaan external programmer. In this manner, the patient's physician maycustomize the treatment for the patient. The patient's physician mayspecify, for example, suitable pacing pulse energy parameter values forparticular pressures. The present invention presents techniques wherebythe patient's physician can relate the pacing of the patient's heart tothe monitored pressures.

[0017] In various embodiments of the present invention, the pressuremonitor and processor function together to continuously monitor apressure in a patient's heart and modulate the energy level of pacingpulses delivered to the heart by a pacemaker as a function of thepressure.

[0018] The present invention has certain advantages. That is, incomparison to known implementations for identifying a need for increasedcardiac output and modulating pacing parameters to increase cardiacoutput to meet this need, various embodiments of the present inventionmay provide one or more advantages. Such advantages may include, forexample, more accurate and less complex determination of need forincreased cardiac output.

[0019] For example, the system and method of the present inventionaccurately determines whether there is a need for increased cardiacoutput by processing a pressure signal that represents pressure in theheart to measure a pressure value indicative of whether an increase incardiac output is needed. By more directly and accurately measuring thesymptoms of cardiac decompensation as reflected in the pressure value,the present invention more effectively treats them. Further, measuringthe pressure value in accordance with the present invention requires aless complicated system and method than existing systems and methods fordetermining whether there is a need for increased cardiac output.Consequently, implementing the present invention requires a lessexpensive device that consumes less power than existing methods. Also,the system and method of the present invention effectively increase thecardiac output as needed by increasing the energy level of pacing pulsesdelivered to the heart by a pacemaker.

[0020] The above summary of the present invention is not intended todescribe each embodiment or every embodiment of the present invention oreach and every feature of the invention. Advantages and attainments,together with a more complete understanding of the invention, willbecome apparent and appreciated by referring to the following detaileddescription and claims taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a schematic view of an implantable medical device.

[0022]FIG. 2 shows the implantable medical device located in and near aheart.

[0023]FIG. 3 is a block diagram illustrating the constituent componentsof an implantable medical device.

[0024]FIG. 4 shows a pacemaker-cardioverter-defibrillator located in andnear a heart.

[0025]FIG. 5 is a functional schematic diagram of one embodiment of animplantable medical device.

[0026]FIG. 6 is a diagram of a system including a pressure monitor and apacemaker.

[0027]FIG. 7 is a graph of voltage amplitude as a function of time,showing pacing pulses of varying energy levels.

[0028]FIG. 8 is a diagram of a human heart, with a pressure sensor and alead.

[0029]FIG. 9 is a timing diagram showing an electrocardiogram signal, asignal indicative of right ventricular pressure, and the first andsecond derivatives of the right ventricular pressure signal.

[0030]FIG. 10 is a timing diagram showing an electrocardiogram signaland a signal indicative of the central venous pressure.

[0031]FIG. 11 is a flow diagram illustrating techniques of theinvention.

[0032]FIG. 12 is a graph showing voltage amplitudes of pacing pulses asa function of an estimated pulmonary artery diastolic pressure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] In the following detailed description of the preferredembodiments, reference is made to the accompanying drawings which form apart hereof, and in which is shown by way of illustration specificembodiments in which the invention may be practiced. It is to beunderstood that other embodiments may be utilized and structural orlogical changes may be made without departing from the scope of thepresent invention. The following detailed description, therefore, is notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims.

[0034]FIG. 1 is a simplified schematic view of one embodiment ofimplantable medical device (“IMD”) 10 of the present invention. IMD 10shown in FIG. 1 is a pacemaker comprising at least one of pacing andsensing leads 16 and 18 attached to connector module 12 of hermeticallysealed enclosure 14 and implanted near human or mammalian heart 8.Pacing and sensing leads 16 and 18 sense electrical signals attendant tothe depolarization and repolarization of the heart 8, and furtherprovide pacing pulses for causing depolarization of cardiac tissue inthe vicinity of the distal ends thereof. The pacing pulses delivered tothe heart each have a particular energy level. The energy level of apacing pulse is a function of several parameters, including pulseamplitude and pulse width. Leads 16 and 18 may have unipolar or bipolarelectrodes disposed thereon, as is well known in the art. Examples ofIMD 10 include implantable cardiac pacemakers disclosed in U.S. Pat. No.5,158,078 to Bennett et al., U.S. Pat. No. 5,312,453 to Shelton et al.,or U.S. Pat. No. 5,144,949 to Olson, all hereby incorporated byreference herein, each in its respective entirety.

[0035]FIG. 2 shows connector module 12 and hermetically sealed enclosure14 of IMD 10 located in and near human or mammalian heart 8. Atrial andventricular pacing leads 16 and 18 extend from connector module 12 tothe right atrium and ventricle, respectively, of heart 8. Atrialelectrodes 20 and 21 disposed at the distal end of atrial pacing lead 16are located in the right atrium. Ventricular electrodes 28 and 29disposed at the distal end of ventricular pacing lead 18 are located inthe right ventricle.

[0036]FIG. 3 shows a block diagram illustrating the constituentcomponents of IMD 10 in accordance with one embodiment of the presentinvention, where IMD 10 is a pacemaker having a microprocessor-basedarchitecture. IMD 10 is shown as including activity sensor oraccelerometer 11, which is preferably a piezoceramic accelerometerbonded to a hybrid circuit located inside enclosure 14 (shown in FIGS. 1and 2). Activity sensor 11 typically (although not necessarily) providesa sensor output that varies as a function of a measured parameterrelating to a patient's metabolic requirements. For the sake ofconvenience, IMD 10 in FIG. 3 is shown with lead 18 only connectedthereto. However, it is understood that similar circuitry andconnections not explicitly shown in FIG. 3 apply to lead 16 (shown inFIGS. 1 and 2).

[0037] IMD 10 in FIG. 3 is most preferably programmable by means of anexternal programming unit (not shown in the figures). One suchprogrammer is the commercially available Medtronic Model 9790programmer, which is microprocessor-based and provides a series ofencoded signals to IMD 10, typically through a programming head whichtransmits or telemeters radio-frequency (RF) encoded signals to IMD 10.Such a telemetry system is described in U.S. Pat. No. 5,312,453 toWyborny et al., hereby incorporated by reference herein in its entirety.The programming methodology disclosed in Wyborny et al.'s '453 patent isidentified herein for illustrative purposes only. Any of a number ofsuitable programming and telemetry methodologies known in the art may beemployed so long as the desired information is transmitted to and fromthe pacemaker.

[0038] As shown in FIG. 3, lead 18 is coupled to node 50 in IMD 10through input capacitor 52. Activity sensor or accelerometer 11 is mostpreferably attached to a hybrid circuit located inside hermeticallysealed enclosure 14 of IMD 10. The output signal provided by activitysensor 11 is coupled to input/output circuit 54. Input/output circuit 54contains analog circuits for interfacing with heart 8, activity sensor11, antenna 56 and circuits for the application of stimulating pulses toheart 8. The rate of heart 8 is controlled by software-implementedalgorithms stored within microcomputer circuit 58.

[0039] Microcomputer circuit 58 preferably comprises on-board circuit 60and off-board circuit 62. Circuit 58 may correspond to a microcomputercircuit disclosed in U.S. Pat. No. 5,312,453 to Shelton et al., herebyincorporated by reference herein in its entirety. On-board circuit 60preferably includes microprocessor 64, system clock circuit 66 andon-board RAM 68 and ROM 70. Off-board circuit 62 preferably comprises aRAM/ROM unit. On-board circuit 60 and off-board circuit 62 are eachcoupled by data communication bus 72 to digital controller/timer circuit74. Microcomputer circuit 58 may comprise a custom integrated circuitdevice augmented by standard RAM/ROM components.

[0040] Electrical components shown in FIG. 3 are powered by anappropriate implantable battery power source 76 in accordance withcommon practice in the art. For the sake of clarity, the coupling ofbattery power to the various components of IMD 10 is not shown in theFigures.

[0041] Antenna 56 is connected to input/output circuit 54 to permituplink/downlink telemetry through RF transmitter and receiver telemetryunit 78. By way of example, telemetry unit 78 may correspond to thatdisclosed in U.S. Pat. No. 4,566,063 issued to Thompson et al., herebyincorporated by reference herein in its entirety, or to that disclosedin the above-referenced '453 patent to Wyborny et al. It is generallypreferred that the particular programming and telemetry scheme selectedpermit the entry and storage of cardiac rate-response parameters. Thespecific embodiments of antenna 56, input/output circuit 54 andtelemetry unit 78 presented herein are shown for illustrative purposesonly, and are not intended to limit the scope of the present invention.

[0042] Continuing to refer to FIG. 3, VREF and bias circuit 82 mostpreferably generates stable voltage reference and bias currents foranalog circuits included in input/output circuit 54. Analog-to-digitalconverter (ADC) and multiplexer unit 84 digitizes analog signals andvoltages to provide “real-time” telemetry intracardiac signals andbattery end-of-life (EOL) replacement functions. Operating commands forcontrolling the timing of IMD 10 are coupled from microprocessor 64 viadata bus 72 to digital controller/timer circuit 74, where digital timersand counters establish the overall escape interval of the IMD 10 as wellas various refractory, blanking and other timing windows for controllingthe operation of peripheral components disposed within input/outputcircuit 54.

[0043] Digital controller/timer circuit 74 is preferably coupled tosensing circuitry, including sense amplifier 88, peak sense andthreshold measurement unit 90 and comparator/threshold detector 92.Circuit 74 is further preferably coupled to electrogram (EGM) amplifier94 for receiving amplified and processed signals sensed by lead 18.Sense amplifier 88 amplifies sensed electrical cardiac signals andprovides an amplified signal to peak sense and threshold measurementcircuitry 90, which in turn provides an indication of peak sensedvoltages and measured sense amplifier threshold voltages on multipleconductor signal path 67 to digital controller/timer circuit 74. Anamplified sense amplifier signal is also provided tocomparator/threshold detector 92. By way of example, sense amplifier 88may correspond to that disclosed in U.S. Pat. No. 4,379,459 to Stein,hereby incorporated by reference herein in its entirety.

[0044] The electrogram signal provided by EGM amplifier 94 is employedwhen IMD 10 is being interrogated by an external programmer to transmita representation of a cardiac analog electrogram. See, for example, U.S.Pat. No. 4,556,063 to Thompson et al., hereby incorporated by referenceherein in its entirety.

[0045] Output pulse generator 96 provides amplified pacing stimuli topatient's heart 8 through coupling capacitor 98 in response to a pacingtrigger signal provided by digital controller/timer circuit 74 each timeeither (a) the escape interval times out, (b) an externally transmittedpacing command is received, or (c) in response to other stored commandsas is well known in the pacing art. By way of example, output pulsegenerator 96 may correspond generally to an output amplifier disclosedin U.S. Pat. No. 4,476,868 to Thompson, hereby incorporated by referenceherein in its entirety.

[0046] As mentioned above, output pulse generator 96 provides eachpacing pulse at a particular energy level. The energy level of a pacingpulse is a function of several parameters, including pulse amplitude andpulse width. As will be described below, the energy level of the pacingpulses provided by output pulse generator 96 can be varied by varyingparameters such as amplitude or width of the pulses. One or more of thecomponents of the microcomputer circuit 58 or controller/timer circuit74 may control the amplitude and width values for the pulses to begenerated by pulse generator 96.

[0047] The specific embodiments of sense amplifier 88, output pulsegenerator 96 and EGM amplifier 94 identified herein are presented forillustrative purposes only, and are not intended to be limiting inrespect of the scope of the present invention. The specific embodimentsof such circuits may not be critical to practicing some embodiments ofthe present invention so long as they provide means for generating astimulating pulse and are capable of providing signals indicative ofnatural or stimulated contractions of heart 8.

[0048] In some preferred embodiments of the present invention, IMD 10may operate in various non-rate-responsive modes, including, but notlimited to, DDD, DDI, VVI, VOO and VVT modes. In other preferredembodiments of the present invention, IMD 10 may operate in variousrate-responsive modes, including, but not limited to, DDDR, DDIR, VVIR,VOOR and VVTR modes. Some embodiments of the present invention arecapable of operating in both non-rate-responsive and rate responsivemodes. Moreover, in various embodiments of the present invention IMD 10may be programmably configured to operate so that it varies the rate atwhich it delivers stimulating pulses to heart 8 in response to one ormore selected sensor outputs being generated. Numerous pacemakerfeatures and functions not explicitly mentioned herein may beincorporated into IMD 10 while remaining within the scope of the presentinvention.

[0049] The present invention is not limited in scope to single-sensor ordual-sensor pacemakers, and is not limited to IMD's comprising activityor pressure sensors only. Nor is the present invention limited in scopeto single-chamber pacemakers, single-chamber leads for pacemakers orsingle-sensor or dual-sensor leads for pacemakers. Thus, variousembodiments of the present invention may be practiced in conjunctionwith one or more leads or with multiple-chamber pacemakers, for example.At least some embodiments of the present invention may be appliedequally well in the contexts of single-, dual-, triple- or quadruple-chamber pacemakers or other types of IMD's. See, for example, U.S. Pat.No. 5,800,465 to Thompson et al., hereby incorporated by referenceherein in its entirety, as are all U.S. Patents referenced therein.

[0050] IMD 10 may also be a pacemaker-cardioverter-defibrillator (“PCD”)corresponding to any of numerous commercially available implantablePCD's. Various embodiments of the present invention may be practiced inconjunction with PCD's such as those disclosed in U.S. Pat. No.5,545,186 to Olson et al., U.S. Pat. No. 5,354,316 to Keimel, U.S. Pat.No. 5,314,430 to Bardy, U.S. Pat. No. 5,131,388 to Pless, and U.S. Pat.No. 4,821,723 to Baker et al., all hereby incorporated by referenceherein, each in its respective entirety.

[0051]FIGS. 4 and 5 illustrate one embodiment of IMD 10 and acorresponding lead set of the present invention, where IMD 10 is a PCD.In FIG. 4, the ventricular lead takes the form of leads disclosed inU.S. Pat. Nos. 5,099,838 and 5,314,430 to Bardy, and includes anelongated insulative lead body 1 carrying three concentric coiledconductors separated from one another by tubular insulative sheaths.Located adjacent the distal end of lead 1 are ring electrode 2,extendable helix electrode 3 mounted retractably within insulativeelectrode head 4 and elongated coil electrode 5. Each of the electrodesis coupled to one of the coiled conductors within lead body 1.Electrodes 2 and 3 are employed for cardiac pacing and for sensingventricular depolarizations. At the proximal end of the lead isbifurcated connector 6 which carries three electrical connectors, eachcoupled to one of the coiled conductors. Elongated coil electrode 5,which is a defibrillation electrode 5, may be fabricated from platinum,platinum alloy or other materials known to be usable in implantabledefibrillation electrodes and may be about 5 cm in length.

[0052] The atrial/SVC lead shown in FIG. 4 includes elongated insulativelead body 7 carrying three concentric coiled conductors separated fromone another by tubular insulative sheaths corresponding to the structureof the ventricular lead. Located adjacent the J-shaped distal end of thelead are ring electrode 9 and extendable helix electrode 13 mountedretractably within an insulative electrode head 15. Each of theelectrodes is coupled to one of the coiled conductors within lead body7. Electrodes 13 and 9 are employed for atrial pacing and for sensingatrial depolarizations. Elongated coil electrode 19 is provided proximalto electrode 9 and coupled to the third conductor within lead body 7.Electrode 19 preferably is 10 cm in length or greater and is configuredto extend from the SVC toward the tricuspid valve. In one embodiment ofthe present invention, approximately 5 cm of the right atrium/SVCelectrode is located in the right atrium with the remaining 5 cm locatedin the SVC. At the proximal end of the lead is bifurcated connector 17carrying three electrical connectors, each coupled to one of the coiledconductors.

[0053] The coronary sinus lead shown in FIG. 4 assumes the form of acoronary sinus lead disclosed in the above cited '838 patent issued toBardy, and includes elongated insulative lead body 41 carrying onecoiled conductor coupled to an elongated coiled defibrillation electrode21. Electrode 21, illustrated in broken outline in FIG. 4, is locatedwithin the coronary sinus and great vein of the heart. At the proximalend of the lead is connector plug 23 carrying an electrical connectorcoupled to the coiled conductor. Elongated coil defibrillation electrode41 may be about 5 cm in length.

[0054] IMD 10 is shown in FIG. 4 in combination with leads 1, 7 and 41,and lead connector assemblies 23, 17 and 6 inserted into connectormodule 12. Optionally, insulation of the outward facing portion ofhousing 14 of IMD 10 may be provided using a plastic coating such asparylene or silicone rubber, as is employed in some unipolar cardiacpacemakers. The outward facing portion, however, may be left uninsulatedor some other division between insulated and uninsulated portions may beemployed. The uninsulated portion of housing 14 serves as a subcutaneousdefibrillation electrode to defibrillate either the atria or ventricles.Lead configurations other that those shown in FIG. 4 may be practiced inconjunction with the present invention, such as those shown in U.S. Pat.No. 5,690,686 to Min et al., hereby incorporated by reference herein inits entirety.

[0055]FIG. 5 is a functional schematic diagram of one embodiment of IMD10 of the present invention. This diagram should be taken as exemplaryof the type of device in which various embodiments of the presentinvention may be embodied, and not as limiting, as it is believed thatthe invention may be practiced in a wide variety of deviceimplementations, including cardioverter and defibrillators which do notprovide anti-tachycardia pacing therapies.

[0056] IMD 10 is provided with an electrode system. If the electrodeconfiguration of FIG. 4 is employed, the correspondence to theillustrated electrodes is as follows. Electrode 25 in FIG. 5 includesthe uninsulated portion of the housing of IMD 10. Electrodes 25, 15, 21and 5 are coupled to high voltage output circuit 27, which includes highvoltage switches controlled by CV/defib control logic 79 via control bus31. Switches disposed within circuit 27 determine which electrodes areemployed and which electrodes are coupled to the positive and negativeterminals of a capacitor bank (which includes capacitors 33 and 35)during delivery of defibrillation pulses.

[0057] Electrodes 2 and 3 are located on or in the ventricle of thepatient and are coupled to the R-wave amplifier 37, which preferablytakes the form of an automatic gain controlled amplifier providing anadjustable sensing threshold as a function of the measured R-waveamplitude. A signal is generated on R-out line 39 whenever the signalsensed between electrodes 2 and 3 exceeds the present sensing threshold.

[0058] Electrodes 9 and 13 are located on or in the atrium of thepatient and are coupled to the P-wave amplifier 43, which preferablyalso takes the form of an automatic gain controlled amplifier providingan adjustable sensing threshold as a function of the measured P-waveamplitude. A signal is generated on P-out line 45 whenever the signalsensed between electrodes 9 and 13 exceeds the present sensingthreshold. The general operation of R-wave and P-wave amplifiers 37 and43 may correspond to that disclosed in U.S. Pat. No. 5,117,824 to Keimelet al., hereby incorporated by reference herein in its entirety.

[0059] Switch matrix 47 is used to select which of the availableelectrodes are coupled to wide band (0.5-200 Hz) amplifier 49 for use indigital signal analysis. Selection of electrodes is controlled bymicroprocessor 51 via data/address bus 53, which selections may bevaried as desired. Signals from the electrodes selected for coupling tobandpass amplifier 49 are provided to multiplexer 55, and thereafterconverted to multi-bit digital signals by A/D converter 57, for storagein random access memory 59 under control of direct memory access circuit61. Microprocessor 51 may employ digital signal analysis techniques tocharacterize the digitized signals stored in random access memory 59 torecognize and classify the patient's heart rhythm employing any of thenumerous signal processing methodologies known to the art.

[0060] The remainder of the circuitry is dedicated to the provision ofcardiac pacing, cardioversion and defibrillation therapies, and, forpurposes of the present invention may correspond to circuitry known tothose skilled in the art. The following exemplary apparatus is disclosedfor accomplishing pacing, cardioversion and defibrillation functions.Pacer timing/control circuitry 63 preferably includes programmabledigital counters which control the basic time intervals associated withDDD, VVI, DVI, VDD, AAI, DDI and other modes of single and dual chamberpacing well known to the art. Circuitry 63 also preferably controlsescape intervals associated with anti-tachyarrhythmia pacing in both theatrium and the ventricle, employing any anti-tachyarrhythmia pacingtherapies known to the art.

[0061] Intervals defined by pacing circuitry 63 include atrial andventricular pacing escape intervals, the refractory periods during whichsensed P-waves and R-waves are ineffective to restart timing of theescape intervals and the pulse widths of the pacing pulses. Thedurations of these intervals are determined by microprocessor 51, inresponse to stored data in memory 59 and are communicated to pacingcircuitry 63 via address/data bus 53. Pacer circuitry 63 also determinesthe amplitude and pulse width of the cardiac pacing pulses under controlof microprocessor 51. By controlling the amplitude and pulse width, IMD10 controls the energy level of the delivered pacing pulses.

[0062] During pacing, escape interval counters within pacertiming/control circuitry 63 are reset upon sensing of R-waves andP-waves as indicated by a signals on lines 39 and 45, and in accordancewith the selected mode of pacing on time-out trigger generation ofpacing pulses by pacer output circuitry 65 and 67, which are coupled toelectrodes 9, 13, 2 and 3. Escape interval counters are also reset ongeneration of pacing pulses and thereby control the basic timing ofcardiac pacing functions, including anti-tachyarrhythmia pacing. Thedurations of the intervals defined by escape interval timers aredetermined by microprocessor 51 via data/address bus 53. The value ofthe count present in the escape interval counters when reset by sensedR-waves and P-waves may be used to measure the durations of R-Rintervals, P-P intervals, P-R intervals and R-P intervals, whichmeasurements are stored in memory 59 and used to detect the presence oftachyarrhythmias.

[0063] Microprocessor 51 most preferably operates as an interrupt drivendevice, and is responsive to interrupts from pacer timing/controlcircuitry 63 corresponding to the occurrence of sensed P-waves andR-waves and corresponding to the generation of cardiac pacing pulses.Those interrupts are provided via data/address bus 53. Any necessarymathematical calculations to be performed by microprocessor 51 and anyupdating of the values or intervals controlled by pacer timing/controlcircuitry 63 take place following such interrupts.

[0064] Detection of atrial or ventricular tachyarrhythmias, as employedin the present invention, may correspond to tachyarrhythmia detectionalgorithms known in the art. For example, the presence of an atrial orventricular tachyarrhythmia may be confirmed by detecting a sustainedseries of short R-R or P-P intervals of an average rate indicative oftachyarrhythmia or an unbroken series of short R-R or P-P intervals. Therate of onset of the detected high rates, the stability of the highrates, and a number of other factors known in the art may also bemeasured at this time. Appropriate ventricular tachyarrhythmia detectionmethodologies measuring such factors are described in U.S. Pat. No.4,726,380 issued to Vollmann, U.S. Pat. No. 4,880,005 issued to Pless etal., and U.S. Pat. No. 4,830,006 issued to Haluska et al., allincorporated by reference herein, each in its respective entirety. Anadditional set of tachycardia recognition methodologies is disclosed inthe article “Onset and Stability for Ventricular TachyarrhythmiaDetection in an Implantable Pacer-Cardioverter-Defibrillator” by Olsonet al., published in Computers in Cardiology, Oct. 7-10, 1986, IEEEComputer Society Press, pages 167-170, also incorporated by referenceherein in its entirety. Atrial fibrillation detection methodologies aredisclosed in Published PCT Application Ser. No. US92/02829, PublicationNo. WO92/8198, by Adams et al., and in the article “AutomaticTachycardia Recognition,” by Arzbaecher et al., published in PACE,May-June, 1984, pp. 541-547, both of which are incorporated by referenceherein in their entireties.

[0065] In the event an atrial or ventricular tachyarrhythmia is detectedand an anti-tachyarrhythmia pacing regimen is desired, appropriatetiming intervals for controlling generation of anti-tachyarrhythmiapacing therapies are loaded from microprocessor 51 into the pacer timingand control circuitry 63, to control the operation of the escapeinterval counters therein and to define refractory periods during whichdetection of R-waves and P-waves is ineffective to restart the escapeinterval counters.

[0066] Alternatively, circuitry for controlling the timing andgeneration of anti-tachycardia pacing pulses as described in U.S. Pat.No. 4,577,633, issued to Berkovits et al., U.S. Pat. No. 4,880,005,issued to Pless et al., U.S. Pat. No. 4,726,380, issued to Vollmann etal., and U.S. Pat. No. 4,587,970, issued to Holley et al., all of whichare incorporated herein by reference in their entireties, may also beemployed.

[0067] In the event that generation of a cardioversion or defibrillationpulse is required, microprocessor 51 may employ an escape intervalcounter to control timing of such cardioversion and defibrillationpulses, as well as associated refractory periods. In response to thedetection of atrial or ventricular fibrillation or tachyarrhythmiarequiring a cardioversion pulse, microprocessor 51 activatescardioversion/defibrillation control circuitry 79, which initiatescharging of high voltage capacitors 33 and 35 via charging circuit 69,under the control of high voltage charging control line 71. The voltageon the high voltage capacitors is monitored via VCAP line 73, which ispassed through multiplexer 55 and in response to reaching apredetermined value set by microprocessor 51, results in generation of alogic signal on Cap Full (CF) line 77 to terminate charging. Thereafter,timing of the delivery of the defibrillation or cardioversion pulse iscontrolled by pacer timing/control circuitry 63. Following delivery ofthe fibrillation or tachycardia therapy microprocessor 51 returns thedevice to cardiac pacing mode and awaits the next successive interruptdue to pacing or the occurrence of a sensed atrial or ventriculardepolarization.

[0068] Several embodiments of appropriate systems for the delivery andsynchronization of ventricular cardioversion and defibrillation pulsesand for controlling the timing functions related to them are disclosedin U.S. Pat. No. 5,188,105 to Keimel, U.S. Pat. No. 5,269,298 to Adamset al., and U.S. Pat. No. 4,316,472 to Mirowski et al., herebyincorporated by reference herein, each in its respective entirety. Anyknown cardioversion or defibrillation pulse control circuitry isbelieved to be usable in conjunction with various embodiments of thepresent invention, however. For example, circuitry controlling thetiming and generation of cardioversion and defibrillation pulses such asthat disclosed in U.S. Pat. No. 4,384,585 to Zipes, U.S. Pat. No.4,949,719 to Pless et al., or U.S. Pat. No. 4,375,817 to Engle et al.,all hereby incorporated by reference herein in their entireties, mayalso be employed.

[0069] Continuing to refer to FIG. 5, delivery of cardioversion ordefibrillation pulses is accomplished by output circuit 27 under thecontrol of control circuitry 79 via control bus 31. Output circuit 27determines whether a monophasic or biphasic pulse is delivered, thepolarity of the electrodes and which electrodes are involved in deliveryof the pulse. Output circuit 27 also includes high voltage switcheswhich control whether electrodes are coupled together during delivery ofthe pulse. Alternatively, electrodes intended to be coupled togetherduring the pulse may simply be permanently coupled to one another,either exterior to or interior of the device housing, and polarity maysimilarly be pre-set, as in current implantable defibrillators. Anexample of output circuitry for delivery of biphasic pulse regimens tomultiple electrode systems may be found in the above-cited patent issuedto Mehra and in U.S. Pat. No. 4,727,877 to Kallok, hereby incorporatedby reference herein in its entirety.

[0070] An example of circuitry which may be used to control delivery ofmonophasic pulses is disclosed in U.S. Pat. No. 5,163,427 to Keimel,also incorporated by reference herein in its entirety. Output controlcircuitry similar to that disclosed in U.S. Pat. No. 4,953,551 to Mehraet al. or U.S. Pat. No. 4,800,883 to Winstrom, both incorporated byreference herein in their entireties, may also be used in conjunctionwith various embodiments of the present invention to deliver biphasicpulses.

[0071] Alternatively, IMD 10 may be an implantable nerve stimulator ormuscle stimulator such as that disclosed in U.S. Pat. No. 5,199,428 toObel et al., U.S. Pat. No. 5,207,218 to Carpentier et al., or U.S. Pat.No. 5,330,507 to Schwartz, or an implantable monitoring device such asthat disclosed in U.S. Pat. No. 5,331,966 issued to Bennet et al., allof which are hereby incorporated by reference herein, each in itsrespective entirety. The present invention is believed to find wideapplication to any form of implantable electrical device for use inconjunction with electrical leads.

[0072]FIG. 6 shows a system 100 illustrating an embodiment of theinvention in which pressure is used to modulate the energy level ofpacing pulses delivered by a pacemaker. System 100, or any of itsconstituent components, could be implanted in a human being or a mammal.System 100 includes pacemaker 114, which paces heart 8. Pacemaker 114 iscoupled to atrial lead 116 and ventricular lead 120. Electrodes 118 and122 disposed on leads 116 and 120 may serve to sense electrical signalsand to pace heart 8. Pacemaker 114 may further be coupled to lead 124,which includes defibrillation coil electrode 126. Alternatively,defibrillation coil electrode 126 may be coupled to lead 116 or 120.Pacemaker 114 may be one of the many forms of implantable medicaldevices 10 described above, or may be an external pacemaker. Atrialelectrode 118 may correspond to any of electrodes 9, 13, 20 or 21described above, ventricular electrode 122 may correspond to any ofelectrodes 2, 3, 28 and 29 described above, and defibrillation coilelectrode 126 may correspond to elongated coil electrode 5 describedabove.

[0073] Pacemaker 114 can deliver pacing pulses to the heart 8 viaelectrodes 118 and 122 at a variety of different energy levels. Theenergy level of a pacing pulse is, in part, a function of the pulseamplitude and pulse width. Therefore, in some embodiments of the presentinvention, the energy level of pacing pulses delivered by pacemaker 114may be changed by varying, among other things, the amplitude or width ofthe pacing pulses.

[0074] Many commercially available pacemakers can produce pulses withamplitudes within the range from 0.5 to 5.0 volts, and widths within therange from 0.05 to 1.5 ms. Some pacemakers can produce pulses withamplitudes as high as 10 volts and widths as wide as 2.0 ms. The size ofthe amplitude and width ranges of pacemaker 114 are less important tothe practice of the present invention, and may be subject to variation,so long as there is some amplitude or width range within which thepacing pulse energy level may be varied for effectiveness.

[0075]FIG. 7 shows examples of three pacing pulses, 140, 142 and 144,that might be delivered by pacemaker 114. FIG. 7 is provided to furtherillustrate that the energy level of a pacing pulse can be varied in atleast two ways in accordance with the present invention. Both pulses,142 and 144, have a higher energy level than pulse 140. Pulse 142delivers more energy to the heart 8 than pulse 140 because of a greateramplitude. Pulse 144 delivers more energy to the heart 8 than pulse 140because of a greater pulse width.

[0076] Pacemaker 114 may also include a microcomputer circuit 58,controller/timer circuit 74, microprocessor 51, pacer timing and controlcircuit 63, or an equivalent device that may control the amplitude,pulse width and other energy parameters of the pulses to be delivered toheart 8. The microcomputer circuit 58, controller/timer circuit 74,microprocessor 51, pacer timing and control circuit 63, or other devicecan receive control signals, signals from other components and/orprogramming, and set the amplitude, pulse width and other parametersaccordingly.

[0077] The present invention presents techniques for adjusting theenergy level of pacing pulses based on the pressure of the blood flowinginside the patient's heart 8. System 100, as shown in FIG. 6, includespressure monitor 102, which is coupled to pressure sensor 106 by lead104. Pressure sensor 106 responds to the absolute pressure inside heart8.

[0078]FIG. 8 is a diagram of a human heart, including a pressure sensorand a lead. Pressure sensor 106 may, as shown in FIG. 8, be placedinside right ventricle 152 of heart 8. Sensor 106 is coupled to lead104, which extends from right ventricle 152, through rightatrioventricular valve 164, and through superior vena cava 176. Lead 104extends further through the patient's circulatory system, eventuallyexiting the circulatory system and coupling to implanted pressuremonitor 102 (not shown in FIG. 8). Pressure monitor 102 may be implantedin the patient's upper chest near pacemaker 114, or in the patient'sabdomen. Alternatively, pressure monitor 102, processor 110 andpacemaker 114 may form a single device implanted in the patient's upperchest.

[0079] Sensor 106 may generate pressure signals itself or may modulatepressure signals conducted through lead 104 along wires 172 and 174. Thepressure signals are a function of the fluid pressure in right ventricle152. Pressure monitor 102 receives, monitors and analyzes the pressuresignals, as will be described in more detail below. An example ofpressure monitor 102 that may be used with this embodiment of thepresent invention is the Chronicle™ Implantable Hemodynamic Monitormanufactured by and commercially available from Medtronic, Inc.

[0080] Pressure sensor 106 may be one of many forms of pressure sensors.One form of pressure sensor that is useful for measuring blood pressureinside a human heart is a capacitive absolute pressure sensor, asdescribed in U.S. Pat. No. 5,564,434 to Halperin, et al., herebyincorporated by reference herein in its entirety. Pressure sensor 106may also be a piezoelectric crystal or piezoresistive pressuretransducer. The invention is not limited to any particular kind ofpressure sensor.

[0081] As will be described below, pressure monitor 102 processes thepressure signals received from pressure sensor 106. Pressure monitor 102may, in some embodiments, identify or calculate a pressure value that isof significance in patient monitoring. As shown in FIG. 6, processor 110receives a signal 108 from pressure monitor 102. Signal 108 may indicatethe pressure value identified or calculated by pressure monitor 102.

[0082] Processor 110 may select a value for one or more pacing pulseenergy parameters, such as pulse amplitude and width, as a function ofsignal 108. If signal 108 indicates a pressure value, processor 110 mayselect the parameter values by comparing the pressure value to a look-uptable of pressure values and associated parameter values. As analternative, processor 110 may select the parameter values by applyingequations that relate pressure values to the parameters. The look-uptable or equations may be stored in memory 132. The look-up table orequations may, for example, be received via remote distribution link128, RF telemetry 130, or from an external programmer.

[0083] Processor 110 may, as shown in FIG. 6, generate a control signal112. Based on control signal 112, processor 110 may control pacemaker114 to adjust the value of one or more pacing pulse energy parameters,so that pacemaker 114 delivers pacing pulses at an adjusted energylevel.

[0084] Although shown in FIG. 6 as logically separate from pressuremonitor 102 and pacemaker 114, processor 110 may be housed insidepressure monitor 102, or inside pacemaker 114. Processor 110 may, forexample, be included in microprocessor 51 in the embodiment of implantedmedical device 10 shown in FIG. 5. Alternatively, processor 110 may beseparate from both pressure monitor 102 and pacemaker 114. Further,pressure monitor 102, pacemaker 114 and processor 110 may be realized asa single implantable device.

[0085] Processor 110 may be implemented as a microprocessor, forexample, or as an ASIC, FPGA, discrete logic circuitry, or analogcircuitry. Processor 110 may execute instructions stored in memory 132,which may comprise any computer-readable medium suitable for storinginstructions, including random access memory (RAM), read-only memory(ROM) non-volatile random access memory (NVRAM), electrically erasableprogrammable read-only memory (EEPROM), flash memory, and the like.

[0086] One pressure of significance in patient monitoring is thepulmonary artery diastolic pressure (PAD). In systole, ventricles 152and 156 (shown in FIG. 8) contract. For a brief period, no blood leavesventricles 152 and 156, and the contraction is isovolumetric. Duringisovolumetric contraction, atrioventricular valves 164 and 170 areclosed by backward pressure differential forces. Aortic valve 168 andpulmonary valve 166 are likewise closed, as the pressure in ventricles152 and 156 is insufficient to force blood through them. Consequently,isovolumetric contraction causes the blood in ventricles 152 and 156 toundergo increasing pressure. In a short time, the pressure in rightventricle 152 overcomes the pressure in pulmonary arteries 158 and 160,drives pulmonary valve 166 open, and ejects blood from right ventricle152 into pulmonary arteries 158 and 160. Similarly, the pressure in leftventricle 156 overcomes the pressure in aorta 162, driving open aorticvalve 168 and ejecting blood into aorta 162. The pressure needed to openaortic valve 168 is normally much higher than the pressure needed toopen pulmonary valve 166.

[0087] The pressure needed to open pulmonary valve 166 is, for practicalpurposes, an accurate measure of the PAD, and is referred to as theestimated pulmonary artery diastolic pressure (ePAD). ePAD closelyreflects the pulmonary capillary wedge pressure, or PCWP, which reflectsthe average pressure in left atrium 154 over a cardiac cycle, alsocalled the mean left atrial pressure or mean LAP. In addition, ePADreflects the filling pressure in the left ventricle 156 during diastole,also called the left ventricular end diastolic pressure or LVEDP. In ahealthy heart, LAP and LVEDP range from approximately 8 mmHg to 12 mmHg.ePAD may be somewhat higher than LAP and LVEDP, but past studiesindicate a strong correlation between ePAD, PCWP, mean LAP and LVEDP. Ina heart having congestive heart failure, each of these pressures may beconsiderably elevated, as will be discussed below.

[0088] Mean LAP and LVEDP are pressures on the left side of heart 8.Practical considerations make it difficult to measure pressures on theleft side of heart 8 directly. These pressures may be measuredindirectly, however, by placing sensor 106 in right ventricle 152 andmeasuring ePAD with pressure monitor 102.

[0089] Measurement of ePAD is not equivalent to measuring the highestpressure in right ventricle 152. During isovolumetric contraction insystole, the pressure in right ventricle 152 increases and forcespulmonary valve 166 open. Pressure in right ventricle 152 does not peakat this point, however. Rather, pressure in right ventricle 152increases during ejection as well, but the pressure increases at areduced rate.

[0090] It is this change in the rate of increase of pressure that helpsidentify ePAD, as illustrated in FIG. 9. Pressure signal 190 from sensor106 in right ventricle 152 is shown in reference to electrocardiogram(ECG) signal 192. ECG signal 192 shows pacing spike 206. ECG signal 192may be sensed by electrodes 118 and/or 122, and may, as shown in FIG. 6,be provided to pressure monitor 102 and processor 110 via pacemaker 114as ECG signal 134.

[0091] R-wave 194 in ECG signal 190 of FIG. 9 represents ventriculardepolarization of heart 8. Following ventricular depolarization,pressure in right ventricle 152 increases, eventually reaching a peakpressure at 196.

[0092] When the pressure in right ventricle 152 overcomes the pressurein pulmonary arteries 158 and 160, pulmonary valve 166 is driven open.When pulmonary valve 166 opens, contraction is no longer isovolumetric.Pressure in right ventricle 152, although still increasing due toventricular contraction, increases at a slower rate. As a result, thereis an inflection point 198 in pressure signal 190 when pulmonary valve166 opens.

[0093] ePAD may be found by determining the pressure at the point onright ventricular pressure singal 190 corresponding to the inflectionpoint 198. Inflection point 198 may be found by taking the firstderivative of right ventricular pressure with respect to time, or dP/dt.Because the slope of pressure signal 190 is at its maximum positivevalue at inflection point 198, the positive peak 208 of dP/dt signal 200corresponds to inflection point 198. Therefore, ePAD may be found byfinding the point on right ventricular pressure signal 190 correspondingto the maximum positive value of dP/dt. Inflection point 198 may also befound by taking the second derivative of right ventricular pressure withrespect to time, or d²P/dt². In this case, ePAD is the pressure at thepoint on right ventricular pressure signal 190 corresponding to thepoint 210 at which signal 202 of d²P/dt² goes negative for the firsttime after R-wave 194. The time at which inflection point 198, peak 208and zero crossing 210 occur is indicated by line 212.

[0094] Pressure monitor 102 may include differentiating circuits thatgenerate d²P/dt² signal 202 and/or dP/dt signal 200. Pressure monitor102 may further include circuits to detect when d²P/dt² signal 202crosses zero in the negative direction after the R-wave, or when dP/dtsignal 200 peaks, both of which occur at line 212. By detecting wheninflection point 198 occurs, pressure monitor 102 may measure thepressure in right ventricle 152 at inflection point 198, which is ePAD.

[0095]FIG. 9 also shows right ventricle pressure signal 190 superimposedon an exemplary pulmonary artery pressure curve 204. As shown in FIG. 9,the point of minimum pulmonary artery pressure 214, the pulmonary arterydiastolic pressure or PAD, is nearly equal to the right ventriclepressure at inflection point 198, when signal 190 and curve 204 crosseach other. The pressure at inflection point 198 is ePAD, the pressureat which the pressure in right ventricle 152 overcomes the pressure inpulmonary arteries 158 and 160, opening pulmonary valve 166. As can beseen, ePAD is a close estimation of PAD.

[0096] ePAD is a significant pressure in many respects. Patients havingchronic congestive heart failure often exhibit elevated ePAD levels. Inparticular, elevated ePAD levels are frequently present in patientshaving advanced cardiac disease and often dilated cardiomyopathy orrestrictive cardiomyopathy. Hearts of patients having congestive heartfailure often fail to achieve adequate circulation, a condition known ascardiac decompensation.

[0097] One factor contributing to cardiac decompensation is pulmonaryedema, in which excess tissue fluid enters the lungs. The fluidaccumulation in the lungs reduces the oxygen-carbon dioxide exchange,leading to an elevation of acid-forming carbon dioxide in the blood.Pulmonary edema is caused by overloading of the heart, i.e., aninability of the heart to expel the blood being returned to it. Whenblood is unable to return to the heart from the pulmonary system, theblood dams up in the lungs. Blood damming up in the lungs leads to anincrease in pulmonary capillary wedge pressure, or PCWP, and results inpulmonary edema.

[0098] Cardiac decompensation and pulmonary edema can be serious. Inmany cases, the conditions require intensive care and hospitalization.Cardiac decompensation and pulmonary edema can be fatal.

[0099] Patients having congestive heart failure are at risk of pulmonaryedema. The damming of the blood in the lungs leads to increased pressurein the pulmonary circulatory system, which results in an elevatedpulmonary artery pressure. Elevated pulmonary artery pressure istherefore a sign of risk of pulmonary edema.

[0100] Because ePAD is a close approximation of pulmonary arterydiastolic pressure, ePAD is also a sign of risk of pulmonary edema. Ingeneral, as a patient's ePAD approaches approximately 25 mmHg, thepatient's risk of pulmonary edema increases. When a patient's ePADexceeds 25 mmHg, pulmonary edema is very likely to occur.

[0101] Another pressure of significance in patient monitoring is thecentral venous pressure, or CVP. The CVP is the pressure in the rightatrium. Methods for sensing and monitoring the CVP are well known in theart, and could include, for example, placing pressure sensor 106 inright atrium 150 or superior vena cava 176 of heart 8.

[0102] In FIG. 10, a CVP signal 220 is shown alongside a correspondingECG signal 222. For each cardiac cycle, the waveform includes an “a”wave 224, which is caused by atrial contraction, a “c” wave 226, whichis caused by the isovolumetric contraction of the ventricle an “x”descent 228, which reflects the atrial relaxation after contraction, a“v” wave 230, which is caused by the atrium filling with blood from thevena cava while the tricuspid valve is closed, and a “y” descent 232,which reflects the decrease in atrial pressure as the tricuspid valveopens and blood flows from the atrium to the ventricle.

[0103] In some embodiments of the present invention, the CVP value atany discrete point during the cardiac cycle may be less important thanchanges observed over time in the average value of the CVP calculatedover a cardiac cycle. The average value of the CVP over a cardiac cycleis referred to as the mean CVP.

[0104] To calculate the mean CVP, pressure monitor 102 could employ anyof the well-known methods in the art for calculating beat-to-beat meanvalues for sensed and sampled physiological signals. Pressure monitor102 may, for example, determine the length of the cardiac cycle bydetecting the R-wave occurrences 234 in the ECG signal 134 provided bypacemaker 114. The mean CVP over that cardiac cycle would then simply bethe average of the values sampled from the signal received from pressuresensor 106 over that cardiac cycle.

[0105] The mean CVP is directly influenced by the function of the rightheart and the pressure of venous blood in the superior and inferior venacava. The mean CVP also indirectly reflects left ventricular function.In heart failure patients, the CVP is higher than normal because as lessblood is pumped into the arterial circulation by the ventricles, bloodbacks up in the venous circulation, which in turn increases the thoracicblood volume. This blood volume increase in the venous circulationincreases venous return, which in turn elevates the mean CVP.Furthermore, the circulatory system will increase venous return, as acompensatory measure to increase the preload to the heart, whenevercardiac output is insufficient.

[0106] Causing more blood to be expelled from the heart, i.e.,increasing cardiac output, would reduce the damming of blood in thelungs and the back up of blood in the venous circulation. Therefore,elevated ePAD and/or elevated mean CVP indicate the need for increasedcardiac output by the ventricles. In addition to ePAD and mean CVP,other cardiac pressures that would indicate a need for increased cardiacoutput may be monitored in the practice of this invention, such as rightventricular systolic pressure, diastolic pressure, pulse pressure, whichis a function of the difference between systolic and diastolicpressures, or the like.

[0107] Cardiac output (CO) is defined as the volume of blood pumped byeach ventricle per minute. CO is determined by two factors: heart rate(HR) in units of beats per minute, and stroke volume (SV) in units ofvolume of blood pumped per stroke, i.e., per beat. The relationshipbetween CO, HR and SV is usually expressed:

CO=HR×SV

[0108] One way to increase CO is to increase SV, i.e., cause theventricles to pump more blood per beat. One technique that has beendiscovered to cause the ventricles to pump more blood per beat is toincrease the energy level of pacing pulses delivered to them bypacemaker 114. Increasing the energy level of a pacing pulse deliveredto a chamber of the heart causes the action potential to traverse thechamber more quickly, which causes the cardiac muscle cells of thatchamber to contract in a more simultaneous manner. This, in turn, causesthe chamber to contract in a more efficient and forceful manner, whichcauses the ventricles to eject more blood per beat.

[0109] For example, one way to increase the energy level of a pacingpulse, as reported by Nakayama, et. al., is to increase the amplitude ofthe pacing pulse as measured in volts. Increasing the voltage amplitudeof a pacing pulse increases field stimulation area, i.e., the number ofcardiac cells simultaneously stimulated by the pulse. Increasing thefield stimulation area thus leaves less area over which the actionpotential must propagate. This causes a more synchronous contraction,which is more efficient and forceful because more cells are contractingin a shorter timeframe. The energy level of a pulse could also bemodified in the practice of the present invention in other ways, such asby increasing the pulse width.

[0110] Another way to cause a ventricle to pump more blood per beat isto fill up the ventricle with more blood before it beats. One way tofill a ventricle up with more blood before it beats is to increase theatrial contribution to ventricular filling by increasing the atrialejection. Increasing the energy level of pacing pulses delivered to theatrium will increase atrial ejection in the same way that increasing theenergy level of pacing pulses delivered to the ventricle increasesventricular ejection.

[0111] The present invention is not limited to pacemakers with aparticular lead configuration. Nor is the present invention limited tomodulating the energy level of pacing pulses delivered to any particularchamber of the heart. The present invention can be practiced with anypacemaker on any chamber or combination thereof. Moreover, the presentinvention is not limited to situations wherein the pacing pulses aredelivered to maintain a proper cardiac rhythm, but can instead bepracticed whenever increased cardiac output is desired.

[0112] Techniques for pressure-based modulation of pacing pulse energylevel are shown in FIG. 11. Pressure monitor 102 monitors a pressure inthe heart via pressure sensor 106 coupled to lead 104 (240). Pressuresensor may, for example, be located in right ventricle 152 or rightatrium 150. Pressure monitor 102 also processes the pressure signal thatit receives from pressure sensor 106 to measure a pressure value (242).Pressure monitor 102 may, for example, differentiate the pressure signalor calculate the mean pressure over a cardiac cycle. Pressure monitor102 may, for example, determine the ePAD or calculate mean CVP usingtechniques described above. Pressure monitor 102 generates a pressuresignal 108 as a function of the measured pressure value, which isreceived by processor 110.

[0113] Processor 110 selects a value for one or more pacing pulse energyparameters as a function of pressure signal 108 (244) and generatescontrol signal 112, which is received by pacemaker 114. As discussedabove, selecting a value for a pacing pulse energy parameter may includeselecting a pulse amplitude or pulse width. Pacemaker 114 deliverspacing pulses with the selected pulse energy parameter values to heart 8as a function of control signal 112 (246). If pressure monitor 102detects an elevated ePAD or elevated mean CVP, for example, processor110 may generate control signal 112 to cause pacemaker 114 to deliverpacing pulses with a higher voltage amplitude, a wider pulse width, orboth. In this way, pacemaker 114 may deliver pacing pulses at a higherenergy level.

[0114] As described above, delivering pacing pulses with a higher energylevel results in a more forceful ejection, thereby increasing cardiacoutput. The results of increasing the cardiac output may be reflected inthe measured pressure value, which may be used to further modify theenergy level of pacing pulses delivered by the pacemaker 114. Thus,system 100 may use feedback continually to monitor pressure in thepatient's heart 8, and adjust the energy level of pacing pulsesdelivered to the heart 8 as a function of the pressure (248). The pacingpulse energy parameter levels can be adjusted for a defined number ofcardiac cycles, on a beat-to-beat basis, a minute-to-minute basis, or onsome other basis.

[0115] Data pertaining to the pressure in a patient's heart, includingthe determined pressure value, may be stored in memory 132. The data mayreflect, for example, the patient's ePAD or mean CVP on a beat-to-beatbasis, a minute-to-minute basis, an hour-to-hour basis, or on some otherbasis.

[0116] This data may thereafter be retrieved via input/output devicessuch as remote distribution link 128 or RF telemetry 130. The data maybe plotted for viewing by a physician. Remote distribution link 128provides a channel for downloading data from the patient over atelephone line or over the internet, for example. RF telemetry 130provides immediate access to the data on a dedicated channel. Typically,a patient is required to visit the physician's office when data are tobe downloaded via RF telemetry 130.

[0117] Input/output devices 128 and 130 allow a person, such as thepatient's physician, to exchange information with processor 110,pressure monitor 102 and pacemaker 114. The information exchanged mayinclude not only pressure data, but also pacing data, patient activitydata, and other numbers, statistics or data.

[0118] The information exchanged may also include programminginstructions. Processor 110 may be programmable by a physician viainput/output devices 128 and 130. Memory 132 may be used to store theinstructions programmed by the physician. The programming may reflect,for example, the physician's judgment as to the appropriate pacing pulseenergy parameter levels over a range of measured pressure values. Theprogramming may also reflect the physician's judgment as to whichchambers should receive pacing pulses with modified energy levels, orhow frequently pacing pulse energy levels should be modified.

[0119] In one embodiment of the present invention, pacing pulse voltageamplitude may be modulated as a function of measured ePAD values. FIG.12 is a graph illustrating an exemplary relationship between ePAD andpacing pulse amplitude in volts. Curve 250 defines the appropriate pulseamplitude as a function of the patient's calculated ePAD. Curve 250 maybe defined by an equation that applies over a range of ePAD values, theequation being of the general form amplitude=f(ePAD).

[0120] As shown in FIG. 12, the pulse amplitude increases non-linearlyas the patient's ePAD approaches 25 mmHg (252). The increase in slope ofcurve 250 represents a rapid increase in pulse amplitude when thepatient is at risk of pulmonary edema. The rapid increase causes SV torise, consequently boosting CO, thereby alleviating the overloading andreducing the risk of pulmonary edema. Although curve 250 defines pulseamplitudes corresponding to an ePAD of about 11 mmHg or greater, thephysician may program pacing pulse energy parameter levels correspondingto any range of determined pressure values.

[0121] The relationships between measured pressure values and pacingpulse energy parameter values may be described as curves, or asequations that defines curves. The physician may also describe thecorrespondence in other ways. The physician may, for example, programdiscrete pulse energy parameter values for discrete measured pressurevalues or ranges of measured pressure values. FIG. 12 shows one suchcorrelation between discrete ePAD values and discrete pulse voltageamplitude values, resulting in a piecewise linear relationship 254. Thesubset of ePAD values between 15 mmHg and 18 mmHg, for example,corresponds to a pulse amplitude of 3.0 volts. Similarly, other subsetsof ePAD values correspond to a single pulse amplitude.

[0122] As another alternative, the correspondence between measuredpressure values and pacing pulse energy parameter values may also bestored in memory 132 as a look-up table that maps pressure values toparameter values. Processor 110 then finds one or more parameter valuescorresponding to the measured pressure value by looking up the pressurevalue in the table.

[0123] The shape of curve 250 and piecewise linear relationship 254shown in FIG. 12 are for purposes of illustration. How pacing pulseenergy parameter levels correspond to determined pressure values dependson the parameter and value at issue. The correspondence may also dependupon the patient's particular needs.

[0124] The preceding specific embodiments are illustrative of thepractice of the invention. It is to be understood, therefore, that otherexpedients known to those skilled in the art or disclosed herein may beemployed without departing from the invention or the scope of theclaims. For example, pacemaker 114 may be responsive to inputs inaddition to pressure-based control signal 112, such as electricalsignals sensed by electrodes 118 and 122, or signals from anaccelerometer.

[0125] The invention further includes within its scope the methods ofmaking and using the systems described above. These methods are notlimited to the specific examples described above, but may be adapted tomeet the needs of a particular patient. These and other embodiments arewithin the scope of the following claims.

[0126] In the claims, means-plus-functions clauses are intended to coverthe recited structures described herein as performing the recitedfunction and not only structural equivalents but also equivalentstructures. Thus, although a nail and a screw may not be structuralequivalents in that a nail employs a cylindrical surface to securewooden parts together, whereas a screw employs a helical surface, in theenvironment of fastening wooden parts a nail and a screw are equivalentstructures.

1. An implantable medical device comprising: a pressure sensor thatgenerates a pressure signal as a function of a pressure in a heart; apressure monitor that identifies a pressure value at a point of maximumslope in the pressure signal; and a processor that causes a pacemaker toadjust the energy level of pacing pulses delivered to the heart as afunction of the pressure value.
 2. The device of claim 1, wherein theprocessor is further configured to generate a control signal as afunction of the pressure value, wherein the pacemaker is configured toreceive the control signal and adjust the energy level of pacing pulsesdelivered to the heart as a function of the control signal.
 3. Thedevice of claim 1, wherein the amplitude of the pacing pulse deliveredby the pacemaker is a function of the pressure value.
 4. The device ofclaim 3, wherein the pulse amplitude is within a range fromapproximately 0.5 volts to approximately 5.0 volts.
 5. The device ofclaim 1, wherein the pulse width of the pacing pulse delivered by thepacemaker is a function of the pressure value.
 6. The device of claim 5,wherein the pulse width is within a range from approximately 0.05 ms toapproximately 1.5 ms.
 7. The device of claim 1, wherein the pressuresensor includes one of a capacitive absolute pressure sensor, apiezoelectric crystal transducer and a piezoresistive pressuretransducer.
 8. The device of claim 1, wherein the pressure monitorcomprises a differentiating circuit, the differentiating circuitconfigured to generate a differential signal that is representative ofthe first derivative of the pressure signal, wherein the pressuremonitor identifies the pressure value corresponding to a maximum pointon the differential signal.
 9. The device of claim 1, wherein thepressure monitor comprises a differentiating circuit, thedifferentiating circuit configured to generate a differential signalthat is representative of the second derivative of the pressure signal,wherein the pressure monitor identifies the pressure value correspondingto a zero crossing point on the differential signal.
 10. The device ofclaim 1, further comprising an input/output device coupled to theprocessor, the input/output device configured to exchange informationbetween a person and the processor.
 11. The device of claim 1, whereinthe device is implanted in the upper chest of a patient.
 12. Animplantable medical device comprising: a pressure sensor that generatesa pressure signal as a function of a pressure in a heart; a pressuremonitor that estimates the pulmonary artery diastolic pressure as afunction of the pressure signal; and a processor that causes a pacemakerto adjust the energy level of pacing pulses delivered to the heart as afunction of the estimated pulmonary artery diastolic pressure.
 13. Thedevice of claim 12, wherein the processor is further configured togenerate a control signal as a function of the estimated pulmonaryartery diastolic pressure, wherein the pacemaker is configured toreceive the control signal and adjust the energy level of pacing pulsesdelivered to the heart as a function of the control signal.
 14. Thedevice of claim 12, wherein the amplitude of the pacing pulse deliveredby the pacemaker is a function of the estimated pulmonary arterydiastolic pressure.
 15. The device of claim 14, wherein the pulseamplitude is within a range from approximately 0.5 volts toapproximately 5.0 volts.
 16. The device of claim 12, wherein the pulsewidth of the pacing pulse delivered by the pacemaker is a function ofthe estimated pulmonary artery diastolic pressure.
 17. The device ofclaim 16, wherein the pulse width is within a range from approximately0.05 ms to approximately 1.5 ms.
 18. The device of claim 12, wherein thepressure sensor is disposed in the right ventricle of the heart.
 19. Thedevice of claim 18, wherein the pressure monitor comprises adifferentiating circuit, the differentiating circuit configured togenerate a differential signal that is representative of the firstderivative of the pressure signal, wherein the pressure monitorestimates pulmonary artery diastolic pressure as a function of thedifferential signal.
 20. The device of claim 18, wherein the pressuremonitor comprises a differentiating circuit, the differentiating circuitconfigured to generate a differential signal that is representative ofthe second derivative of the pressure signal, wherein the pressuremonitor estimates pulmonary artery diastolic pressure as a function ofthe differential signal.
 21. The device of claim 12, further comprisingan input/output device coupled to the processor, the input/output deviceconfigured to exchange information between a person and the processor.22. The device of claim 12, wherein the device is implanted in the upperchest of a patient.
 23. An implantable medical device comprising: apressure sensor that generates a pressure signal as a function of apressure in an atrium of a heart; a pressure monitor to measure apressure value as a function of the pressure signal; and a processorthat causes a pacemaker to adjust the energy level of pacing pulsesdelivered to the heart as a function of the pressure value.
 24. Thedevice of claim 23, wherein the processor is further configured togenerate a control signal as a function of the pressure value, whereinthe pacemaker is configured to receive the control signal and adjust theenergy level of pacing pulses delivered to the heart as a function ofthe control signal.
 25. The device of claim 23, wherein the amplitude ofthe pacing pulse delivered by the pacemaker is a function of thepressure value.
 26. The device of claim 25, wherein the pulse amplitudeis within a range from approximately 0.5 volts to approximately 5.0volts.
 27. The device of claim 23, wherein the pulse width of the pacingpulse delivered by the pacemaker is a function of the pressure value.28. The device of claim 27, wherein the pulse width is within a rangefrom approximately 0.05 ms to approximately 1.5 ms.
 29. The device ofclaim 23, wherein the pressure sensor includes one of a capacitiveabsolute pressure sensor a piezoelectric crystal transducer and apiezoresistive pressure transducer.
 30. The device of claim 23, whereinthe pressure value measured by the pressure monitor is the mean centralvenous pressure.
 31. The device of claim 30, wherein the pressure sensoris disposed in the right atrium.
 32. The device of claim 23, furthercomprising an input/output device coupled to the processor, theinput/output device configured to exchange information between a personand the processor.
 33. The device of claim 23, wherein the device isimplanted in the upper chest of a patient.
 34. A method comprising:identifying a point of maximum slope in a pressure signal, the pressuresignal being a function of a pressure in a heart; identifying a value ofthe pressure signal at the maximum slope of the pressure signal; andadjusting the energy level of pacing pulses delivered to the heart by apacemaker as a function of the identified pressure value.
 35. The methodof claim 34, wherein adjusting the energy level of pacing pulsescomprises adjusting the pulse amplitude of pacing pulses.
 36. The methodof claim 34, wherein adjusting the energy level of pacing pulsescomprises adjusting the pulse width of pacing pulses.
 37. The method ofclaim 34, wherein identifying the point of maximum slope in the pressuresignal comprises: generating the first derivative of the pressure signalduring a cardiac cycle; and sensing the point on the pressure signal atwhich the first derivative of the pressure signal is maximized.
 38. Themethod of claim 34, wherein identifying a point of maximum slope in thepressure signal comprises: sensing an R-wave of a cardiac cycle;generating the second derivative of the pressure signal during thecardiac cycle; and sensing the point on the pressure signal at which thesecond derivative of the pressure signal becomes negative after theR-wave.
 39. The method of claim 34, wherein adjusting the energy levelof pacing pulses delivered to the heart by a pacemaker as a function ofthe pressure value comprises: storing a range of pressure values and oneor more pacing pulse energy parameter values corresponding to eachpressure value in the range; selecting the pacing pulse energy parametervalues that correspond to the identified pressure value; and deliveringpacing pulses to the heart, wherein the energy level of the deliveredpacing pulses is a function of the selected pacing pulse energyparameter values.
 40. The method of claim 39, further comprising:receiving a range of pressure values; and receiving one or morecorresponding pacing pulse energy parameter values for each pressurevalue in the range.
 41. The method of claim 40, further comprisingstoring the received range of pressure values and the correspondingpacing pulse energy parameter values in memory.
 42. The method of claim39, wherein the pressure values correspond to the pacing pulse energyparameter values according to an equation that defines a curve.
 43. Amethod comprising: estimating the pulmonary artery diastolic pressure asa function of a pressure within a heart; and adjusting the energy levelof pacing pulses delivered to the heart by a pacemaker as a function ofthe estimated pulmonary artery diastolic pressure.
 44. The method ofclaim 43, wherein adjusting the energy level of pacing pulses comprisesadjusting the pulse amplitude of pacing pulses.
 45. The method of claim43, wherein adjusting the energy level of pacing pulses comprisesadjusting the pulse width of pacing pulses.
 46. The method of claim 43,wherein estimating the pulmonary artery diastolic pressure as a functionof the pressure in a heart comprises estimating the pulmonary arterydiastolic pressure as a function of the pressure in the right ventricle.47. The method of claim 43, wherein adjusting the energy level of pacingpulses delivered to the heart by a pacemaker as a function of theestimated pulmonary artery diastolic pressure comprises: storing a rangeof pulmonary artery diastolic pressure values and one or more pacingpulse energy parameter values corresponding to each pulmonary arterydiastolic pressure value in the range; selecting the pacing pulse energyparameter values that correspond to the estimated pulmonary arterydiastolic pressure value; and delivering pacing pulses to the heart,wherein the energy level of the delivered pacing pulses is a function ofthe selected pacing pulse energy parameter values.
 48. The method ofclaim 47, further comprising: receiving a range of pulmonary arterydiastolic pressure values; and receiving one or more correspondingpacing pulse energy parameter values for each pulmonary artery diastolicpressure value in the range.
 49. The method of claim 48, furthercomprising storing the received range of pulmonary artery diastolicpressure values and the corresponding pacing pulse energy parametervalues in memory.
 50. The method of claim 47, wherein the pulmonaryartery diastolic pressure values correspond to the pacing pulse energyparameter values according to an equation that defines a curve.
 51. Amethod comprising: determining a mean value of a pressure signal over aperiod of time, the pressure signal being a function of a pressurewithin a heart; and adjusting the energy level of pacing pulsesdelivered to the heart by a pacemaker as a function of the mean value.52. The method of claim 51, wherein adjusting the energy level of pacingpulses comprises adjusting the pulse amplitude of pacing pulses.
 53. Themethod of claim 51, wherein adjusting the energy level of pacing pulsescomprises adjusting the pulse width of pacing pulses.
 54. The method ofclaim 51, wherein determining a mean value of the pressure signal over aperiod of time comprises: determining the length of a cardiac cycle;sampling the pressure signal over the length of the cardiac cycle; andcalculating the mean of the pressure signal over the length of thecardiac cycle.
 55. The method of claim 51, wherein determining a meanvalue of a pressure signal over a period of time, the pressure signalbeing a function of the pressure within a heart, comprises determining amean value of a pressure signal over a period of time, the pressuresignal being a function of the pressure within the right atrium of aheart, wherein the determined mean value is the mean central venouspressure.
 56. The method of claim 51, wherein adjusting the energy levelof pacing pulses delivered to the heart by a pacemaker as a function ofthe mean pressure value comprises: storing a range of mean pressurevalues and one or more pacing pulse energy parameter valuescorresponding to each mean pressure value in the range; selecting thepacing pulse energy parameter values that correspond to the determinedmean pressure value; and delivering pacing pulses to the heart, whereinthe energy level of the delivered pacing pulses is a function of theselected pacing pulse energy parameter values.
 57. The method of claim56, further comprising: receiving a range of mean pressure values; andreceiving one or more corresponding pacing pulse energy parameter valuesfor each mean pressure value in the range.
 58. The method of claim 57,further comprising storing the received range of mean pressure valuesand the corresponding pacing pulse energy parameter values in memory.59. The method of claim 56, wherein the mean pressure values correspondto the pacing pulse energy parameter values according to an equationthat defines a curve.
 60. A computer-readable medium comprisinginstructions that cause a processor to: identify a point of maximumslope in a pressure signal, the pressure signal being a function of apressure within a heart; identify a value of the pressure signal at themaximum slope of the pressure signal; and adjust the energy level ofpacing pulses delivered to the heart by a pacemaker as a function of theidentified pressure value.
 61. The computer-readable medium of claim 60,wherein the instructions that cause a processor to identify a point ofmaximum slope in a pressure signal further comprise instructions thatcause a processor to: generate the first derivative of the pressuresignal during a cardiac cycle; and sense the point on the pressuresignal at which the first derivative of the pressure signal ismaximized.
 62. The computer-readable medium of claim 60, wherein theinstructions that cause a processor to identify a point of maximum slopein a pressure signal further comprise instructions that cause aprocessor to: sense an R-wave of a cardiac cycle; generate the secondderivative of the pressure signal during the cardiac cycle; and sensethe point on the pressure signal at which the second derivative of thepressure signal becomes negative after the R-wave.
 63. Thecomputer-readable medium of claim 60, wherein the instructions thatcause a processor to adjust the energy level of pacing pulses deliveredto the heart by a pacemaker as a function of the identified pressurevalue further comprise instructions that cause a processor to: store arange of pressure values and one or more pacing pulse energy parametervalues corresponding to each pressure value in the range; select thepacing pulse energy parameter values that correspond to the identifiedpressure value; and direct the pacemaker to adjust the energy level ofpacing pulses delivered to the heart, wherein the energy level of thepacing pulses is a function of the selected pacing pulse energyparameter values.
 64. A computer-readable medium comprising instructionsthat cause a processor to: receive an estimated pulmonary arterydiastolic pressure value; and adjust the energy level of pacing pulsesdelivered to the heart by a pacemaker as a function of the estimatedpulmonary artery diastolic pressure value.
 65. The computer-readablemedium of claim 64, wherein the instructions that cause a processor toadjust the energy level of pacing pulses delivered to the heart by apacemaker as a function of the estimated pulmonary artery diastolicpressure value further comprise instructions that cause a processor to:store a range of pulmonary artery diastolic pressure values and one ormore pacing pulse energy parameter values corresponding to eachpulmonary artery diastolic pressure value in the range; select thepacing pulse energy parameter values that correspond to the estimatedpulmonary artery diastolic pressure value; and direct the pacemaker toadjust the energy level of pacing pulses delivered to the heart, whereinthe energy level of the pacing pulses is a function of the selectedpacing pulse energy parameter values.
 66. A computer-readable mediumcomprising instructions that cause a processor to: determine a meanvalue of the pressure signal over a period of time, the pressure signalbeing a function of a pressure within a heart; and adjust the energylevel of pacing pulses delivered to the heart by a pacemaker as afunction of the mean pressure value.
 67. The computer-readable medium ofclaim 66, wherein the instructions that cause a processor to determine amean value of the pressure signal over a period of time, furthercomprise instructions that cause a processor to: determine the length ofa cardiac cycle; sample the pressure signal over the length of thecardiac cycle; and calculate the mean of the pressure signal over thelength of the cardiac cycle.
 68. The computer-readable medium of claim66, wherein the instructions that cause a processor to adjust the energylevel of pacing pulses delivered to the heart by a pacemaker as afunction of the mean pressure value, further comprise instructions thatcause a processor to: store a range of mean pressure values and one ormore pacing pulse energy parameter values corresponding to each meanpressure value in the range; select the pacing pulse energy parametervalues that correspond to the determined mean pressure value; and directthe pacemaker to adjust the energy level of pacing pulses delivered tothe heart, wherein the energy level of the pacing pulses is a functionof the selected pacing pulse energy parameter values.
 69. An implantablemedical device comprising: means for identifying a point of maximumslope in a pressure signal, the pressure signal being a function of apressure within a heart; means for identifying a value of the pressuresignal at the maximum slope of the pressure signal; means for adjustingthe energy level of pacing pulses delivered to the heart as a functionof the identified pressure value.
 70. An implantable medical devicecomprising: means for estimating the pulmonary artery diastolic pressureas a function of a pressure within a heart; and means for adjusting theenergy level of pacing pulses delivered to the heart as a function ofthe estimated pulmonary artery diastolic pressure value.
 71. Animplantable medical device comprising: means for determining a meanvalue of a pressure signal over a period of time, the pressure signal afunction of a pressure within a heart; and means for adjusting theenergy level of pacing pulses delivered to the heart as a function ofthe determined mean pressure value.